This invention relates generally to integration and packaging of radio-frequency (RF) and millimeter-wave circuit components and, more particularly, to techniques for electrically isolating RF and millimeter-wave components, to maintain circuit performance. The difficulty designers face in this area is that parallel RF paths less than & substrate width apart have reduced RF signal transfer efficiency because RF energy is lost through coupling to the adjoining RF path. The invention is concerned with providing electrical or electromagnetic isolation in a lateral direction, that is, in directions parallel to the plane of the substrate. Isolation in a direction perpendicular to the substrate is provided conventionally by the metal backplane on one face of the substrate, and by a layer of metalization on the upper surface. To solve the isolation problem, module designers may separate the circuit components by many substrate widths, but this approach greatly increases module size and weight. Another approach is to compartmentalize each module by machining cavities in it to separate the components. Again, this approach adds complexity and cost to the module.
Yet another approach is to form via holes through the RF module, and to electroplate the holes to form a piece-wise ground plane that reduces inter-component coupling. The large number of holes required for this approach greatly reduces the module's mechanical integrity, because of the substrate material between holes tends to crack if the hole spacing is too small. The use of via holes also limits the possibilities for integration into other RF systems. Further, the presence of multiple via holes does not provide complete isolation, especially for higher signal frequencies. As a rule of thumb, the via holes must be spaced by closer than a quarter wavelength to provide effective isolation. Even when this closely spaced, the via holes do not provide a barrier to isolation for second and higher harmonics of the RF signals. Some designs have used multiple lines of via holes to provide additional isolation, but this technique takes even more space on the module and further weakens the substrate material. In addition, none of the foregoing approaches results in a compact module, contrary to the desirable goal of reducing the size and weight of the module.
It will be appreciated from the foregoing that there is still a need for a new approach to provide lateral electrical isolation of RF and millimeter-wave circuit components mounted close together on a substrate. The present invention satisfies this need.